The present application is based on Japanese Patent Application No. 2001-177510, which is incorporated herein by reference.
1. Field of the Invention
The present invention relates to an electromagnetic wave suppressor sheet having adequate electric insulation properties and an electromagnetic wave suppressing function.
2. Description of the Related Art
Generally, the problem of EMC (Electro-Magnetic Compatibility) has been manifest in electric equipment. Measures against the problem have been taken from the two points of view of emission of unnecessary electromagnetic waves and durability. In recent years, with the advance of miniaturization and higher functions of information communication equipment, semiconductor devices have a tendency to further increase their operating frequencies in order to have higher performance. Particularly with digital signals processed at a higher rate, amplitude values of the signals become lower in order to reduce power consumption. Thus, there is increasing a risk that even weak radio-frequency noise causes a malfunction.
For such a reason, covering a semiconductor package with a metal cap or mounting a filter on a printed wiring circuit board has been heretofore carried out as one measure against this risk. However, since an extra mounting space is required, the mounting density of electronic parts cannot increase satisfactorily. Accordingly, other solutions have been investigated. Nowadays, from such a background, an electromagnetic wave shielding technique suitable for high-density mounting is demanded. For example, as proposed in Unexamined Japanese Patent Publication No. Hei. 9-35927 (hereinafter referred to as xe2x80x9crelated artxe2x80x9d), a method in which an electromagnetic wave suppressor sheet is installed near the place where unnecessary electromagnetic waves are generated in electronic equipment has been known as one of methods for absorbing unnecessary electromagnetic waves of the electronic equipment and shielding the electronic equipment. The electromagnetic wave suppressor sheet is obtained as follows. That is, soft magnetic material particles which have a large imaginary part xcexcxe2x80x3 of complex permeability (hereinafter referred to as xe2x80x9cimaginary part permeabilityxe2x80x9d) are mixed and dispersed into synthetic resin, and formed into the electromagnetic wave suppressor sheet.
Some of magnetic particles heretofore used to be mixed and dispersed in electromagnetic wave suppressor sheets have a low insulation resistance value. However, an electromagnetic wave suppressor sheet manufactured with the use of a material in which such magnetic particles having a low insulation resistance value have been mixed and dispersed as filler at a high ratio may be placed directly on lead pins of a semiconductor package or placed on conductors of a printed wiring circuit board. In such a case, there has been a risk of short-circuit. If there does not occur short-circuit, the electric insulation resistance value of the electromagnetic wave suppressor sheet becomes so small that there has been a risk of so-called signal crosstalk in which an electric signal in one signal line leaks into another signal line.
For such a reason, in order to secure the electric insulation properties, the related art discloses a technique in which an oxidation process such as a vapor-phase slow oxidation process or a liquid-phase slow oxidation process is applied to soft magnetic particles to be used so that an oxide film is formed on the surfaces of the soft magnetic particles so as to make the soft magnetic particles nonconductive. However, the surface resistance value of the electromagnetic wave suppressor sheet obtained in the related art is 107-108 xcexa9. Though there is an effect in reducing unnecessary radiation, it has to be said that this technique is not satisfactory as a measure against short-circuit or crosstalk when the electromagnetic wave suppressor sheet is installed on lead pins of a semiconductor package.
From such a point of view, the magnetic particles filled into an electromagnetic wave suppressor sheet at a high ratio are required to have not only a large imaginary part permeability xcexcxe2x80x3 but also electric insulation properties.
The invention was developed in such circumferences. An object of the invention is to provide an electromagnetic wave suppressor sheet which can reduce unnecessary radiation of electromagnetic waves from electronic equipment at a high level and which has not only a large imaginary part permeability xcexcxe2x80x3 but also excellent electric insulation properties.
In order to attain the foregoing object, an electromagnetic wave suppressor sheet according to the invention is designed to be formed into a sheet-like shape out of a material made of synthetic resin or rubber in which powder of conjugated magnetic particles surface-treated with an insulating inorganic material has been dispersed.
That is, in order to obtain an electromagnetic wave suppressor sheet using magnetic loss of magnetic particles and excellent in unnecessary radiation reduction properties as a measure against electromagnetic interference, the inventor et al. conducted a series of investigations chiefly into electric insulation properties of magnetic particles used as filler of the electromagnetic wave suppressor sheet. As a result, the inventor et al. found that when conjugated magnetic particles surface-treated with an insulating inorganic material were used, an electromagnetic wave suppressor sheet having sufficiently high volume resistivity and extremely improved electric insulation properties could be obtained. Thus, the inventor et al. achieved the invention.
Features and advantages of the invention will be evident from the following detailed description of the preferred embodiments described in conjunction with the attached drawings.